1. Field of the Invention
This invention relates to demodulators, and more particularly, to digital demodulators for decoding phase-modulated carrier waves.
2. Description of the Prior Art
Apparatus for transmitting digital data over communications channels by means of phase-modulation techniques is well known in the art. This apparatus has typically comprised such circuit elements as high, low, and band-pass filters, delay lines, amplifiers, and product modulators, all adapted for use with analog waves, for example, as disclosed in U.S. Pat. Nos. 3,128,342 and 3,128,343 issued on Apr. 7, 1964 to P. A. Baker.
The recent development of low-cost digital integrated circuit elements has brought forth the possibility that the modulating and demodulating functions could be accomplished by combinations of such circuit elements with greater reliability and at lower cost than with the analog elements used in the prior art. Demodulators that incorporate some digital circuit elements for decoding purposes have been disclosed in the prior art, for example, see U.S. Pat. No. 3,590,381 issued to Robert G. Ragsdale on Mar. 17, 1969, and U.S. Pat. No. 3,746,995 issued to H. C. Schroeder et al. on July 17, 1973. Both these patents disclose demodulators comprising such analog elements as modulators and filters in combination with digital circuit elements.
It is desirable to provide a digital demodulator that could be fabricated by the techniques used to produce large-scale integrated circuits, without the need for numerous additional discrete components. Thus, it is desirable to provide a digital demodulator that would comprise digital circuit elements almost exclusively. An example of such a digital demodulator is disclosed in U.S. Pat. No. 3,729,684 to Donald G. Shuda.
It is, therefore, an object of this invention to provide improved apparatus for demodulating a received phase-modulated wave that substantially comprises digital circuit elements.